This invention relates to gas separation membranes having porous defects that are sealed with caulking particles so as to improve gas separation performance of the membranes. More specifically, it relates to a process of healing the defects in asymmetric gas separation membranes by contacting the skin of the membrane with an aqueous suspension of curable polymeric caulking particles, removing water of suspension and curing the polymer of the particles to seal the defects.
Gas separation membranes of selectively gas permeable materials are well known and commercially important devices for separating the components of gas mixtures in many industries. These membranes can have many physical forms such as tubes and flat, pleated or spiral wound films. Membranes in the form of small diameter hollow fibers are particularly valued mainly because they can be assembled in bundles within modules that provide very high gas transfer surface area within extraordinarily small module volume.
Because thinner membranes provide higher the membrane flux than thicker membranes of the same selectively gas permeable composition, there is a strong incentive to make the selectively gas permeable portion (sometime herein, xe2x80x9cthe active layerxe2x80x9d) of hollow fiber membranes ultra thin. A favored technique involves forming an asymmetric structure in which the active layer is a thin, dense stratum positioned coextensively adjacent to a somewhat porous and usually much thicker stratum, occasionally referred to herein as a xe2x80x9csupport layerxe2x80x9d. When positioned on a surface of the membrane, the active layer is sometimes referred to as a xe2x80x9cskinxe2x80x9d. The support layer provides negligible selectivity because the gas mixture to be separated passes freely through the porous structure. Beneficially, flow through the support layer is less restricted because the pores provide little resistance to transmembrane gas flow despite the comparatively greater support layer thickness. As the thickness of the active layer is typically about a few hundred to a few thousand angstroms, an asymmetric membrane structure provides an overall good balance of gas flow and selectivity.
In striving to reduce the thickness of the active layer, most fabricators use methods which make the active layer so thin that porous defects normally form in the layer. Defects can also be formed as a consequence of handling the hollow fiber during fabrication steps such as washing, dehydration, drying and bobbin winding. While it is difficult to quantify these defects, they may be generally characterized as being less than about 50 Angstroms in size.
If not repaired, the defects provide passages directly through the selectively gas permeable material. These defects thus dramatically reduce the membrane selectivity compared to selectivity that would be obtained from a continuous layer of the selectively permeable material. Defects occupying typically only as much as about 10xe2x88x925-10xe2x88x926 fraction of the membrane surface can render a membrane useless for selective gas separation purposes.
Conventional methods of gas separation membrane manufacture sometimes call for so-called xe2x80x9cpost-treatmentxe2x80x9d processing in which the defects are effectively caulked with certain sealant materials. Generally such post-treatment procedures involve dissolving a polymeric sealant in an organic solvent, causing the resulting solution to at least partially coat the defective membrane surface and penetrate the porous defects. The solvent is removed leaving the polymeric solute occluding the pores.
U.S. Pat. No. 4,230,463 discloses methods of coating porous hollow fiber membranes with liquid coating material to provide a separation factor greater than either that of the uncoated hollow fiber membrane or the coating material. The methods call for dipping the porous membrane in the undiluted liquid coating material or in a solution of the liquid coating material dissolved in a hydrocarbon solvent.
EP 0 242 069 teaches a method of forming a semipermeable composite membrane by applying an aqueous emulsion of curable silicone (polyorganosiloxane) onto a porous substrate, evaporating the water, coalescing the siloxane particles and crosslinking the siloxane to form a continuous film of polyorganosiloxane over the porous substrate. A surfactant can be used to stabilize the polymer precursor droplets in the aqueous emulsion The method is useful for forming separation membranes on organic solvent sensitive substrates. It does not suggest to caulk defects in a pre-formed ultra thin separation membrane.
Japanese Patent Publication JP 57094307 discloses the procedure of contacting a dry composite semipermeable membrane with an aqueous solution of generally amphoteric surfactant or nonionic surfactant to increase the water permeability of the dry composite membrane.
Japanese Patent Publication JP 1123617 discloses the method of eliminating minute defects in composite membranes of a selectively gas permeable layer deposited on a sheet or hollow yam porous substrate. The method calls for applying surfactants having at most 30 dynes/cm surface tension, and preferably bearing perfluoroalkyl groups onto the membrane. The surfactant can be dissolved in water and the membrane is dipped in the surfactant solution to contact the selectively gas permeable layer with the surfactant.
U.S. Pat. No. 4,575,385 discloses asymmetric gas separation membranes treated with certain aromatic permeation modifiers. A disclosed method of treating the membrane calls for dipping a hollow fiber membrane for about 10 seconds in an aqueous methanol solution containing 2 wt. % of the permeation modifier while applying vacuum to the bore of the hollow fiber.
U.S. Pat. No. 5,034,024 discloses the process of improving permselectivity of asymmetric aromatic polyimide gas separation membranes entailing contacting the membrane with a nonsolvent mixture for the membrane, preferably including a swelling agent, and containing a surfactant. The use of nonionic amine ethyloxylate and alcohol ethoxylate surfactants dissolved in aqueous solutions is disclosed.
EP 0 107 636 teaches that an anisotropic porous substrate, preferably a hollow fiber, can be coated with the condensation product of silanol terminated poly(dimethylsiloxane) and a specified cross linking agent having particularly identified characteristics.
EP 0 179 474 discloses treating an aromatic polyamide or amide-like polymer membrane with a dilute solution of a cationic surfactant in a volatile non-polar organic solvent of low surface tension to provide improved selectivity with respect to gas separation of two gases selected from among carbon dioxide, methane and helium.
U.S. Pat. No. 4,472,175 discloses the method of contacting one or both surfaces of a preformed selectively gas permeable asymmetric separation membrane with an effective amount of a Brxc3x8nsted-Lowery acid and thereby increasing the separation factor of the membrane relative to the separation factor exhibited prior to such contacting.
U.S. Pat. No. 4,484,935 teaches the method of preparing a multicomponent selectively gas permeable hollow fiber membrane having preferably a porous polysulfone substrate onto which is coated a condensation product of silanol-terminated poly(dimethyl siloxane) and a permeation modifying radical selected from among 15 specified radicals. The coating is applied to the substrate in the form of a solution of the coating material dissolved in a solvent such as pentane.
In U.S. Pat. No. 4,486,376 it is disclosed to treat a porous polymeric gas separation membrane with a volatile liquid containing a modifying agent. The modifying agent is either a volatile organic liquid capable of dissolving at least 1% of the porous membrane at 25xc2x0 C. or a volatile organic liquid capable of causing the porous membrane to shrink by at least 3%.
U.S. Pat. No. 4,634,531 discloses the method of treating a semipermeable membrane in two steps, firstly with an aqueous solution of a water soluble compound, primarily polyfunctional amines, and secondly with an aqueous solution of another water soluble compound, primarily polyfunctional aldehydes, the two compounds being such that when contacted with each other they make water insoluble or very slightly water soluble materials. The membranes being treed consist of cross-linked polymers based on furfuryl alcohol monomer and also polyamide and polyurea membranes. The membranes are largely used for reverse osmosis utilities.
U.S. Pat. No. 4,751,104 teaches a method of preparing gas separation membranes which includes coating a porous support membrane with an emulsion or solution of a polymer dissolved in a solvent and a plasticizer for the polymer. Preferably the emulsion or solution is drawn into the porous support membrane under vacuum to increase penetration and the solvent is evaporated at elevated temperature. Fluorinated hydrocarbon solvents are disclosed.
In U.S. Pat. No. 4,863,496 it is taught to improve the selectivity of asymmetric gas separation membranes having a wide range of hole sizes by controlled application of a reactive monomer within or on the surface of the membrane. The monomer can be diluted in a noninteracting fluid. After application, the monomer is reacted to form a polymer effectively sealing defects and imperfections in the membrane. Monomers in organic solvents are disclosed.
Despite many efforts, it is still desirable to have a very effective gas separation membrane that provides the combination of high permeance and high selectivity for gas mixtures and especially for separating oxygen and nitrogen gas mixtures. To consistently and reliably fabricate such a membrane, it is desired to have a procedure that permits repair of minute porous defects of a thin active separation layer without significantly increasing active layer thickness and thereby decreasing permeate flux. That is, a suitable repair method would offer the ability to make an active layer so thin that it unavoidably contains correctable defects. There also thus remains a great need for a method of post treating a pre-formed selectively permeable gas separation membrane to repair a very low surface concentration of minute porous defects in an otherwise continuous very thin active, selectively permeable layer of a gas separation membrane. Moreover, it is desirable to have a porous defect repair method that utilizes an aqueous medium and preferably avoids use of volatile organic compounds which can be hazardous to handle or environmentally repugnant to dispose of.
Accordingly, the present invention provides a method of repairing porous defects in a selectively gas permeable portion of a gas separation membrane comprising
providing a suspension comprising caulking particles of a curable polymer dispersed in an aqueous solution,
contacting the portion of the membrane having the porous defects with the suspension,
removing water of the suspension from the membrane, and
heating the caulking particles and membrane to an elevated temperature for a duration effective to cure the polymer, thereby causing the polymer to seal the porous defects.
The invention further provides a method of repairing porous defects in the selectively gas permeable portion of a gas separation membrane which is an asymmetric hollow fiber having an annular cross section comprising a porous part and a substantially nonporous selectively gas permeable part radially adjacent to the porous part and in which the step of removing water comprises drying the membrane directly to a dry state without replacing the water with a non-aqueous liquid prior to obtaining the membrane in the dry state.
This invention additionally provides a hollow fiber gas separation membrane having an asymmetric structure defined by an annular cross section comprising a porous part and a substantially nonporous selectively gas permeable part radially adjacent to the porous part in which the selectively gas permeable part comprises a plurality of porous defects occluded by cured caulking particles of a curable polymer.
There is also provided a gas separation membrane having an asymmetric structure defined by a dense selectively gas permeable skin comprising sealed porous defects the membrane being formed by a method comprising
providing a suspension comprising caulking particles of a curable polymer dispersed in an aqueous solution,
contacting a portion of the membrane having porous defects with the suspension,
removing water from the suspension, and
heating the caulking particles and membrane to an elevated temperature for a duration effective to cure the polymer, thereby causing the polymer to seal the porous defects.